Steel slag is discharged in a very large quantity from a steelmaking furnace such as a converter, an open-hearth furnace or an electric arc furnace. Effective utilization or disposal of said steel slag is therefore one of the important tasks in the field of steelmaking.
A typical chemical composition, as expressed in weight percentage, of a steel slag, of example, a top-blowing oxygen converter slag, is as follows:
CaO: 50.4%
SiO.sub.2 : 15.3% PA1 Al.sub.2 O.sub.3 : 1.00% PA1 Total Fe: 16.3% PA1 P.sub.2 o.sub.5 : 2.28% PA1 TiO.sub.2 : 1.66% PA1 MnO: 5.21% PA1 MgO: 2.01% PA1 V.sub.2 o.sub.5 : 1.21% PA1 CaO: 68 to 72%, PA1 SiO.sub.2 : 22 to 26%, PA1 Al.sub.2 O.sub.3 : 1 to 3%, PA1 Fe.sub.2 O.sub.3 : 0.2 to 1.0%, PA1 P.sub.2 o.sub.5 : 0.1 to 0.6%, PA1 TiO.sub.2 : 0.4 to 0.9%, PA1 MnO: trace to 0.4%, PA1 MgO: 0.3 to 3.0%, PA1 CaF.sub.2 : 0.3 to 2.0%,
when such a steel slag is employed as a subbase course material or an aggregate (hereinafter simply called "aggregate"), said aggregate is always susceptible of disintegration under the effect of free lime (free CaO) and dicalcium silicate (2CaO.SiO.sub.2) contained in said steel slag. In addition, because a steel slag contains considerable Fe constituents as mentioned above, the aggregate produced therefrom has a higher specific gravity than natural aggregates. For these reasons, steel slags are very rarely utilized as aggregates, but mostly disposed of as waste for reclamation at present.
In order to dispose of these large quantities of steel slag in the form of wastes for reclamation, however, vast places for reclamation are required, and the transportation thereof requires a substantial cost. Development of a method for effectively utilizing a steel slag only increasing in amount is therefore one of the important tasks requiring an urgent solution in the field of steelmaking.
Under such circumstances, many trials have been made to use a steel slag as a material for cement. For example, there have been proposed several methods, all based on basic ideas of bringing the chemical composition closer to that of the conventional Portland cement, which comprise, for example, adding a steel slag to a blast furnace cement comprising a blast furnace water-granulated slag and a Portland cement, or manufacturing a normal Portland cement from a steel slag. However, because a steel slag contains considerably phosphorus (P) and manganese (Mn) constituents which impair the strength of cement, the use of a steel slag in a large quantity as a material for cement cannot be expected even by any of the methods mentioned above.
In Japanese Patent Provisional Application No. 83,693/74, a method for manufacturing an alumina cement from a steel slag is proposed, which comprises reducing P.sub.2 O.sub.5, MnO and FeO contained in a molten steel slag with carbon contained in a molten pig iron by bringing said molten steel slag added with alumina in contact with said molten pig iron and stirring them, thereby removing such constituents as phosphorus, manganese and iron by causing transfer of said constituents from said molten steel slag into said molten pig iron. This method however requires a large quantity of molten pig iron and much heat. In this method, furthermore, as mentioned above, such constituents detrimental to cement as phosphorus and manganese transfer from a molten steel slag into a molten pig iron. In order to reuse said molten pig iron in steelmaking, therefore, it is necessary to remove these detrimental constituents, and this requires much time and money together with many other difficulties.